1. Field of the Invention
The present invention generally relates to an image forming apparatus such as a copier, a facsimile machine, a printer, or the like applying an electrophotographic method to form an image by using toner, and more particularly to operations control of a developing device.
2. Description of the Related Art
A developing device included in an image forming apparatus applying an electrophotographic method develops and visualizes an electrostatic latent image formed on an image carrier by using a developer including two components: toner and a carrier. In the developing device, in which a developing process ends in a developing area and the toner is consumed, the developer is collected and is used again for developing an image after being mixed and stirred with replenished toner. The developer used for the developing device is needed to maintain a constant toner density and charge quantity in order to acquire a stable toner image. The toner image is adjusted with the consumed toner for image formation and the replenished toner. The charge quantity is given by a frictional charge when the toner is mixed with the carrier. In the developing device using the developer including the two components, the toner is sufficiently mixed with the carrier to achieve a uniform toner density distribution, and also the toner image is stabilized by charging the toner.
In a general-purpose developing device, the toner is distributed and charged by using a stirring effect of a rotation of two screws within a short time until the replenished toner is pumped up to a developing roller. Especially, in a case of consuming a large amount of toner, the replenished toner is pumped up to the developing roller before being sufficiently distributed. Accordingly, image quality is degraded due to toner scattering or the like.
To solve this problem, a developing device, in which stirring performance is improved by separating a developing part from a developer stirring part, is known. Advantageously, in this developing device, compared with a developing device which conducts stirring and conveying by using a screw, an amount of toner which is not charged or not sufficiently charged becomes smaller, and toner scattering and background fouling occur less. However, since the developing part and the developer stirring part are separately arranged, a developer circulation part for circulating the developer is needed between the developing part and the developer stirring part. As the developer circulation part, Japanese Patents No. 3734096 and No. 3349286 disclose conveying mechanism using an air pump or a mohno pump.
In a case of successively carrying the developer in the tube by using the air, a conveyance amount may fluctuate depending a state of the developer. For example, powder characteristics of the developer are changed depending on a deterioration level, temperature, moisture, and the like of the developer. When a developer capacity of the developing part is decreased due to a decrease of the conveyance amount of the developer, the developer amount to be pumped up to the developing roller becomes insufficient and an image defect occurs. Especially, in a case in which a one-way circulation system is applied to the developing device, the developer is supplied to the developing roller from a supply screw arranged parallel to the developing roller, and the developer being supplied is carried to a collection screw after an imaging operation. The developer amount in a vicinity of the supply screw becomes less toward a downstream side of a flow direction of the developer (a volume decreases more). Accordingly, when the conveyance amount of the developer decreases, a depletion state occurs, in which the developer is not supplied to the developing roller at the downstream side in the flow direction of the developer. To prevent the occurrence of the depletion state beforehand, it may be considered to control the conveyance amount of the developer by detecting the developer amount in the vicinity of the supply screw.
Accordingly, a powder surface and the volume of the developer in the imaging device may be detected, and the conveyance amount of the developer may be controlled based on a detection result. Japanese Laid-open Patent Applications No. H08-36294 and No. 2009-198967 disclose a developing device using a piezoelectric oscillation element or a magnetic permeability detection method.
In the above-described technologies, the piezoelectric oscillation element is used to detect whether the powder surface of the developer is higher than a predetermined height. However, the supply screw and the collection screw are provided. Thus, in the developing device in which the developer supplied from the supply screw to the developing roller is not supplied again to the developing roller, the powder surface of the developer at a downstream side of the supply screw is significantly lower than that at an upstream side of the supply screw. The powder surface fluctuates along a slope of a screw, and thus, is not constant. Thus, these problems make an output of the powder surface unstable and a detection of the powder surface becomes difficult. Also, in a case in which a magnetic permeability sensor is used as a powder surface detection part, since both the powder surface and the toner density change, the powder surface is not detected if the toner density is not accurately recognized.
As described above, the developing device included in the image forming apparatus of the electrophotographic method is known in which the developing part and the developer stirring part are separately arranged and a stirring performance is improved. Since the developing part and the developer stirring part are separately arranged, developer circulation part for circulating the developer is needed between the developing part and the developer stirring part. As the developer circulation part, the conveying mechanisms using the air pump or the mohno pump are known.
A developer conveyance by air is influenced by bulk density, fluidity, toner density, and the like of the developer and the conveyance amount fluctuates. A developer balance between the developing part and the developer stirring part is changed. Especially, when the conveyance amount decreases and a developer capacity of the developing part decreases, a volume of the developer for the screw to be supplied to the developing roller is decreased, and the developer is not sufficiently supplied to the developing roller. An amount of the developer being pumped up is decreased at a downstream side of the developing roller after the developer passes a doctor blade. As a result, an image density irregularity may occur. Also, due to the decrease of the developer capacity of the developing part, an amount of the developer of the developer stirring part increases. Thus, a torque driving a stirring member is increased and a load becomes greater. Accordingly, it is needed to control a circulation amount of the developer to be constant, to stabilize the pumped-up amount and the developer balance between the developing part and the developer stirring part.
In order to maintain a height of a powder surface of the developer at the downstream side of the supply screw, regardless of an image to be formed, Japanese Laid-open Patent Application No. 2009-47989 discloses a developing device which controls a developer supply amount for the developing part based on a number of pixels to be written, a detection result of the toner density by a toner density detection part, a toner amount to be replenished in the externally arranged developing stirring part, and a driving time of the developing device. This technology is effective to increase and decrease an average conveyance amount due to the toner density and the fluidity of the developer.
However, as illustrated in FIG. 1, the developer conveyance using the air is performed in which the conveyance amount is carried with fluctuation (pulsation) at a shorter interval. When the powder surface is temporarily lowered due to the pulsation of the conveyance amount, the developer conveyance may not be sufficiently performed for this case. It has been known that a scale of the pulsation depends on conveyance conditions such as the fluidity, a supply amount of the air, and the like of the developer. When the supply amount of the air is increased or a rotation of the rotary re-feeder is increased to increase the conveyance amount, the pulsation becomes greater in response to these increases. Also, since the pulsation of the powder surface becomes greater, the powder surface fluctuates. It becomes difficult to accurately detect and control the powder surface.
To correspond to a change of the conveyance amount of the developer due to the pulsation, it is required to detect the powder surface and the volume of the developer in the developing device, and to control the conveyance amount of the developer using the air based on a detection result. Japanese Laid-open Patent Applications No. H08-36294 and No. 2009-198967 disclose a developing device using a piezoelectric oscillation element or a magnetic permeability detection method.